You do not need to flush the ball mill with helium. Nickel resists oxidation at ambient very well, and any surface oxide remaining will be reduced on the first contact with hydrogen.
If this is un-activated Raney-type nickel you are milling, of course you know to activate after ball milling, NOT before. From: Roarty, Francis X Would ball milling nickel in a glove box of helium produce a similar oxygen free powder without the need for a bell jar to out gas?
